摘要:针对2016年5月2日发生在华东地区的一次飑线过程，利用WRF模式进行高分辨率数值模拟。在成功模拟飑线发生、加强和移动的基础上，对此次过程中对流不稳定特征以及引起对流稳定度变化的原因进行诊断分析。结果表明：（1）在降水发生前，低层大气表现为对流不稳定；降水发生后，对流不稳定能量得到释放，大气趋于稳定。为了分析引起对流稳定度变化的原因，推导了局地直角坐标系中相当位温垂直梯度的倾向方程，其中位势散度是引起位势稳定度局地变化的主要强迫项。在弱降水区，低层位势散度为负值，有利于增强位势不稳定；强降水区及其前沿为位势散度正值区，倾向于抑制位势不稳定。在强降水区低层，位势散度的主要分量为垂直风切变项，代表垂直风切变和大气湿斜压性的综合作用；高层的主要分量为散度项，代表水平散度和位势稳定度的耦合作用。（2）位势散度能综合表征降水区上空垂直风切变、大气湿斜压性、水平辐合辐散和大气位势稳定度变化等特征，因而与降水联系紧密。本文利用位势散度对飑线降水进行预报，结果表明，位势散度与小时观测降水在时间和空间上吻合较好，对降水区有一定的指示意义，可以为飑线降水业务预报提供参考。
关键词:飑线/
对流不稳定/
位势稳定度/
位势散度
Abstract:The Weather Research and Forecast (WRF) model with a high resolution is used to simulate the rainfall during a squall line process that occurred in eastern China on 2 May 2016. Based on the successful simulation of the squall line process including its formation, strengthening, and evolution, the convective instability characteristics and the causes of the convective stability variation are analyzed. The results are as follows. (1) Before the rainfall occurrence, the atmosphere was convectively unstable; after the rainfall occurred, however, the atmosphere tended to be stable due to the release of unstable convective energy. In order to explore the physical cause of the convective stability, the tendency equation of vertical potential temperature gradient in a local rectangular coordinate system is deduced and the potential divergence is the major forcing term in the equation. In the weak rainfall area, the potential divergence in the lower atmosphere was negative, which was conductive to enhanced potential instability. In the strong precipitation area, the potential divergence was positive and tended to inhibit the potential instability. In the lower levels of rainfall area, the main component of the potential divergence was the vertical wind shear item, which represented the joint effects of the vertical wind shear and the atmospheric moist baroclinicity. While in the higher levels, the main component was the divergence item, which reflected the coupling effects of horizontal divergence and potential stability. (2) Since the potential divergence can well represent the effects of vertical wind shear, atmospheric baroclinicity, horizontal divergence, and the variation of atmospheric potential stability, it is used to forecast precipitation during the squall line process in the present study. Results show that the spatial and temporal distribution of potential divergence is in good agreement with that of observed hourly precipitation. Therefore, the potential divergence is indicative of rainfall area and can provide an important reference for operational precipitation forecast during squall line processes.
Key words:Squall/
Convective instability/
Potential stability/
Potential divergence
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